1. Field of the Invention
The present invention relates to cables for interfacing audio signal sources and loads in an audio system in general and in particular to single-ended and balanced digital signal cables for propagating audio signals in a digital format between a source of digitized audio signals and a load, such as a compact disk player, digital-to-analog and analog-to-digital converters, a digital audio tape (DAT) recorder, and the like.
2. Description of the Related Art
As digital components, e.g. compact disk players, digital-to-analog (D/A) and analog-to-digital (A/D) converters, digital audio tape (DAT) players, and the like, have been added to otherwise conventional analog signal audio systems, the cables used for interconnecting the digital components have generally been the same type of cables used for interconnecting the analog signal components. However, the use of analog signal cables for this purpose has been found to be unsatisfactory.
The requirements for interfacing digital components in an audio system have been found to be stringent because of the presence in such components of circuits that provide very fast signal-level transitions, i.e. risetimes, e.g. 10 nanoseconds (ns), and have very wide bandwidths, e.g. 20 to 50 megahertz (MHz).
As a consequence, it has been found that conventional analog signal cable, even analog signal cable designed for use in very expensive, high quality analog audio signal systems, typically cannot adequately propagate digital signals. One of the reasons for this is believed to be due to the relatively low capacitive impedance of conventional analog signal cables.
It has been found that among the disadvantages of an analog signal cable which has a relatively high capacitive impedance are the disadvantages that, as the capacitive impedance decreases, the bandwidth of the cable may become undesirably narrow and the risetime of the signals applied to the cable may become undesirably long, particularly if the signals are digital signals. When the signals are digital signals an undesirably narrow bandwidth and an undesirably long signal risetime can result in signal jitter in the circuit or system component receiving the signals. This is due to the fact that the circuit or system component receiving the signals has a difficult time determining if and when the received signal has made a transition between a logical low and a logical high, i.e. "1" or "0". To a listener of music, for example, signal jitter is heard as distortion of the music or other audio signal or as a lack of clarity in the reproduction of the detail in the source material.
Another disadvantage of using a cable designed for propagating analog signals to interconnect digital components in an audio system is related to impedance matching which, while important in interfacing analog components, is even more important in interfacing digital components. If the output impedance of one digital component is not matched to the input impedance of another, the high frequency, high-speed digital signal will reflect at the point of impedance change. These reflections cause errors in the detection of the music or other audio signal by the receiving unit, creating "false" signals. Once again, a listener will hear or interpret the false signals as a lack of clarity or distortion in the music or other audio signal. But, even more serious is the fact that these anomalies can, if they are severe enough, create actual drop-out of the source material data, which is heard as audible and disturbing distortions in the harmonic relationships in the material.
Another disadvantage of conventional cables designed for propagating analog signals and, for that matter, conventional cables which are currently being used for propagating digital signals is that none of the conventional cables now being used is tuned or comprises means for being tuned to the bandwidth of typical digital signal sources now being used in audio systems. This is important because, if a digital signal cable is tuned or comprises means for being tuned to the bandwidth of a digital signal source, the performance of the cable can be maximized with a minimum of unwanted coupled noise.